1. Field of the Invention
The present invention relates to a technology for a power source system for supplying power to a device, and in particular, to a technology for a power source system provided with a fuel cell.
2. Description of the Related Art
Fuel cells are devices for generating electrical energy from fuel and oxidant and are capable of providing high power generation efficiency. One of the main features of fuel cells is that electric power is generated through direct power generation, in contrast to conventional power generation methods in which electric power is generated via a thermal or kinetic energy process. Therefore, small-scale fuel cells can also be expected to provide high power generation efficiency. Furthermore, since fuel cells discharge less nitrogen compounds and the like and generate less noise and vibration, the negative influence on the environment can be reduced. As mentioned above, since fuel cells can effectively utilize the chemical energy contained in fuel and have environmentally friendly characteristics, they are expected to serve as energy supply systems in the 21st century. Therefore, fuel cells have received attention as promising new power generation systems usable in portable devices, automobiles, space devices, and the like, i.e., usable in various power generation applications from small to large scale. Thus, the technological development of fuel cells for practical use is in full progress.
Among the various types of fuel cells, polymer electrolyte fuel cells are characterized by lower operational temperatures and higher power densities than other types of fuel cells. A direct methanol fuel cell (hereinafter abbreviated as “DMFC”) is a form of polymer electrolyte fuel cell and has received particular attention in recent years. In the DMFC, methanol serving as fuel is directly supplied to an anode without modification to obtain electric power through the electrochemical reaction of the methanol with oxygen. In this electrochemical reaction, carbon dioxide serving as a reaction product is discharged from the anode, and product water serving as a reaction product is discharged from the cathode. As compared to hydrogen, methanol has higher energy per unit volume, is suitable for storage and easy to handle. Thus, DMFCs are expected to be employed as a power source for automobiles, portable devices (such as cellular phones, notebook type personal computers, PDAs, MP3 players, digital cameras, and electronic dictionaries (books)), and the like.
Techniques for replacing conventional dry batteries with such fuel cells have been proposed. For example, a fuel cell is known which is composed of an inner case for containing the fuel cell and a plastic or metal outer case provided outside the inner case and having the same shape as a general-purpose chemical battery.
Moreover, a power source system is known which is configured such that the output voltage of the power generation module thereof for generating power using fuel varies with time in a manner corresponding to the discharge characteristics of conventional voltaic batteries.
Meanwhile, oxidant such as air must be stably supplied to a fuel cell. When air supply is not stable or air is deficient, the output characteristics of the fuel cell are unstable. In particular, when a fuel cell is used as a substitute for a dry battery used in various devices, the area around the air electrode (cathode) of the fuel cell mounted on a device of a particular type is not always in an open state. As described above, a fuel cell having the same shape as a D size dry battery has been known. However, in this case, an air circulation hole for supplying air to the fuel cell is provided in the surface on which a positive terminal having the same shape as a positive terminal of the dry battery is provided. In this structure, the area around the air electrode is not open. Therefore, deficiency of air may occur, and therefore the output characteristics of the fuel cell may become unstable.